Near-field communication, known to those skilled in the art by the abbreviation NFC, is a wireless connectivity technology which enables communications over a short distance, for example 10 cm, between electronic devices, such as for example contactless smartcards or mobile telephones emulated in card mode, and readers.
NFC technology is particularly suited to connecting any type of user device and enables fast and easy communications.
A contactless object is an object capable of exchanging information via an antenna with another contactless object, for example, a reader, according to a contactless communications protocol.
An NFC object, which is a contactless object, is an object compatible with NFC technology.
NFC technology is an open technology platform standardized in the ISO/IEC 18092 and ISO/IEC 21481 standards, but incorporates many already-existing standards, such as for example the type A and type B protocols defined in the ISO-14443 standard which can be communications protocols usable in the NFC technology.
Aside from its conventional function of a telephone, a cellular mobile telephone may be used (if it is equipped with specific circuitry) for exchanging information with another contactless device, for example a contactless reader, using a contactless communications protocol usable in the NFC technology.
This allows information to be exchanged between the contactless reader and secure elements situated within the mobile telephone. Numerous applications are thus possible such as mobile ticketing in public transport (the mobile telephone behaves as a travel ticket) or else mobile payments (the mobile telephone behaves as a payment card).
During a transmission of information between a reader and an object emulated in tag or card mode, the reader generates a magnetic field using its antenna which is generally, in the standards conventionally used, a sine wave at 13.56 MHz. The intensity of the magnetic field is in the range between 0.5 and 7.5 amps/meter RMS (Root Mean Square).
On the other side, the antenna of the object emulating the tag modulates the field generated by the reader.
This modulation is carried out by modifying the load connected to the terminals of the antenna of the object.
By modifying the load across the terminals of the antenna of the object, the output impedance of the antenna of the reader changes owing to the magnetic coupling between the two antennas. This results in a change in the amplitudes and/or the phases of the voltages and currents present on the antennas of the reader and of the object. Accordingly, in this way, the information to be transmitted from the object to the reader is transmitted via load modulation to the antenna current of the reader.
The variation in load effected during the load modulation results in an amplitude and/or phase modulation of the signal (voltage or current) on the antenna of the reader. A copy of the antenna current is generated and injected into the receiver chain of the reader where it is demodulated and processed so as to extract the information transmitted.
The best transfer of power between the reader and the tag is obtained when the tag is equipped with a circuit matched with the resonant circuit of the reader, and itself resonant at the frequency of the signal transmitted by the reader, for example 13.56 MHz.
However, when the tag is too close to the reader, the resonance frequency of the resonant circuit of the reader will shift towards another value of frequency owing to the magnetic coupling between the two resonant circuits of the reader and of the tag. This results accordingly in a reduction of the efficiency and hence of the power transmitted by the reader which may, in certain cases, lead to a loss of the link between the reader and the tag.
Similarly, when the tag is very far from the reader, and even if the resonance frequency of the resonant circuit of the tag is equal to the emission frequency (13.56 MHz, for example), this will result in a modification of the resonance frequency of the resonant circuit of the reader, albeit to a lesser extent, owing to the reduction in the magnetic coupling between the reader and the tag.